Electrolytic bright polishing



Patented July 29, 1947 ELECTROLYTIC BRIGHT POLISHING Harold I. White,Middletown, Ohio, assignor to The American Rolling-Mill Company,Middletown, Ohio, a corporation of Ohio No Drawing. Application April29, 1940, Serial No. 332,238

claims. (01. 204-140) i This invention relates to a process ofelectrolytic bright polishing and to a polishing bath which has beenfound advantageous in the bright polishing of various metals and alloys.In the past the most commonly used methods of polishing metals involvedmechanical abrasion and bufling. These methods are expensive due to thecost of the equipment used, the hand labor involved, and, in the case ofgrinding, the high metal loss. They are dimcult to apply to intricateshapes, and often cause a dust hazard.

One worker has proposed an electrochemical smoothing and polishing ofmetallic surfaces in which the metal is made an anode in an electrolyteof concentrated phosphoric acid. While this process satisfactorilycleans most metal surfaces, it fails to produce a degree of brightnessor luster, particularly on certain of the so-called stainless steels,which would enable it to replace entirely mechanical polishing andbufling.

It is an object of the present invention to provide an electrolyticpolishing method involving a maximum brightening of the surfaces of alarge variety of metals including all types of stainless steels, and inparticular that type of stainless steel containing approximately 17% ofchromium. It is a further object to accomplish this brightening with alower weight loss, and to provide a bath which has superior throwingpower. For example, I have polished intricate shapes of 17-chromestainless steel to a uniform finish comparable to that obtained bybuffing, with a weight loss of only approximately 0.068 ounce per squarefoot which corresponds to about 0.0001 inch removed from the thicknessof the pieces.

It is an object-of my invention to provide a process by which a highpolish can be produced without the necessity of mechanical buiiing, andon objects which cannot readily be buifed on account of their complexityof design or lack of strength. Another object of my invention is the(provision of a method of producing a polish on stainless steelscomparable or equal to that produced by a buffing operation, without thedisadvantage of bufling, lying in the production of a cold-workedsurface layer containing some ferritic iron with its inferior corrosionresistance.

It is'an object of my invention to provide a novel electrolyte whichwill have very good throwing power and which will have a long life.Another object of my invention is the provision of a method of brightpolishing which is not only advantageous for all of the various types ofstainless steels, but which also is advantageous 2 steels, nickel,copper, brass, Monel metal, canmium and other metals and alloys whichare low in non-metallics, and which together with their reactionproducts are soluble in the electrolyte.

These and other objects of my invention which will be pointed out inmore detail hereinafter, or which will be apparent to one skilled in theart upon reading these specifications, I accomplish by that series ofmethod steps and by that composition of matter of which I shall nowdescribe certain exemplary embodiments.

Briefly, in the practice of my invention I provide an electrolyticpolishing bath composed of concentrated phosphoric acid diluted by oneor more soluble organic compounds having properties similar to those ofnormal butyl alcohol.

The work piece to be polished is made an electrode in this bath, theanode in the case of direct current, though alternating current can beused, and an electric current passed through it until the desiredbrightening is obtained.

While not wishing to be limited by theory I believe the action of theorganic compound is to reduce the ion concentration of the electrolyte.Tests seem to indicate that for best results the polishing efiect shouldbe brought about largely by the action of the electric current and thatdirect chemical attack by the electrolyte itself should be minimized.Dilution of the electrolyte with water would increase the ionization andresult in a pitted surface having markedly decreased luster. Suitableorganic compounds reduce the ion concentration and result in improvedluster.

All organic compounds are not equally efiective in producing thisresult, and I have determined the properties which the compounds mustpossess to be suitable. Compounds which are too volatile, such asalcohols with fewer than three carbon atoms per molecule, should not beused, since they are very rapidly 10st from the bath by evaporation.While in some cases their addition produces a better result thanphosphoric acid alone, the luster is definitely inferior to thatproduced by the higher alcohols. On the other hand it, is necessary thatthe organic compounds be soluble in phosphoric acid, and compounds suchas are typified bythe long-chain alcohols cannot be used. Octyl alcohol,for example, has a sufiiciently long chain of carbon atoms to be lowenough in solubility in phosphoric acid to decrease its effectiveness inproducing brightening to a considerable extent.

Of the many organic compounds which I have in connection with aluminum,silicon steel, mild tried, the following have been found particularlystainless steels.

quantities of other compounds-will sometimes be permissible; forexample, those which might occur in using impure mixtures of organiclay-products.

While for simplicity we prefer to use the pure compounds, it may bedesirable for the sake of economy to use them in the impure or mixedstate, and it can readily be determined by the worker whether theaddition of a given amount of one or more impurities to the preferredcomposition will be detrimental to the production of my result.

The most desirable proportions of phosphoric acid and the organiccompound may vary somewhat with the material being treated. As examplesof particular compositions which have proved satisfactory for certaintypes of metals,

and without limitation, I cite the following: On'

ll-chrome steel, 1. e., a stainless steel containing in the neighborhoodof 17% chromium, I have found that above of normal butyl alcohol(commercial) by volume with 95% of 85% phosacid with 25 parts of butylalcohol gave nearly as bright a polish as the 50-50 composition of 85%phosphoric acid. These compositions work equally well on the 18-8stainless steel, 1. e., a grade of stainless steel containing in theneighborhood of 18% chromium and 8% nickel; With this grade, however, aswell as with other chromium-nickel stainless steels such as 25-12, it isnot essential that the alcohol content be so high to obtain the optimumpolish, as is the case with l7-chrome steel. I have found that with thistype of stainless steel a mixture of 10 parts of normal butyl alcoholwith 90 parts of 85% phospl oric acid produces a satisfactory brightnessand results in an extremely low metal loss. -'I'he proportions ofphosphoric acid and organic compound are generally not critical and canreadily be determined by the operator to produce the maximum' lusterwith the minimum current and metal loss. It should be noted that it ispreferable to use a grade of acid low in fluorides. The mixture shouldbe well stirred after pouring together to insure uniform composition.

The temperature of the bath does not appear to be very critical, andsatisfactory brightening temperatures. The optimum temperature can betemperatures for long periods of time some of the organic compound willevaporate.

The current density does not appear to be highly critical. I haveemployed direct current of densities from one eighth to ten amperes persquare inch of surface brightened, and good results were obtained atcurrent densities 'over this entire range with the 50-50 phosphoricacid, butyl alcohol bath. Generally speaking, high current densitieswill produce brightening in somewhat less time than the lower densities.I generally prefer to use from one-half to five amperesp square inch.

Although I have described my process as being applicable to the use ofdirect current, the work piece being made the anode, this is notnecessarily so and in many instances a satisfactory brightening can beobtained by using alternating current. In this case both electrodes canbe the work pieces being polished. In polishing some metals, for exampleild steel. a very bright polish can be obtained using alternatingcurrent. With others, for example l'l-chrome steel, the surface isbrightened but is not as smooth and mirror-like; and the solution tendsto heat up, necessitating artificial cooling. For the most uniformresults on various materials and to produce the highest polish I preferto use direct current.

The time required to produce a given brightness, of course, depends uponthe current density and the original surface of the metal. On certainrefrigerator door parts I have found that a good bright sample wasobtained in two minutes. using a current of one ampere per square inchof surface brightened. Considerable brightness was obtained in one-halfminute and in one minute, although the brightness was not as great asthat produced in two minutes. On the other hand, samples polished forthree to five minutes did not appear much brighter than those polishedfor two minutes.

There is considerable range as to the choice of cathodes and they may bemade of stainless steel, copper or any conductive material, metallic orotherwise, which will not contaminate the bath. Electrodes of carbon,for example, may be employed, but are not necessary.

For best results, but without limitation, it is preferred that thecathode have an area comparable to that of the surface being polishedand that the two surfaces bdsituat'ed roughly parallel to each other andnot too far apart. To polish deeply recessed objects, best results areobtained when a cathode is located within the recess.

The phosphoric acid-alcohol mixture may be held in a tank of somenon-conducting material determined readily for each metal being polishedsuch as crock or glassware, or acid brick. Lead and iron tanks have beenin use for a considerable period of time, although the electrolyteattacks these materials slightly.

This process and polishing bath will work on a number of metals althoughthey will not work on certain metals such as, for example, tin bronze.It is believed that the reason it will not work on tin bronze is thatthe tin forms an insoluble phosphate which interferes with thebrightening reaction. It is not successful on cast iron in some casesbecause of the high amount of graphite which is insoluble. In general ithas proved successful on all metals and alloys which under the influenceof the electric current are not completely insoluble in phosphoric acid,which are lowin metalloids, and which do not form temperature, since ifthe bath is held at high in oluble reaction products with phosphoricacid.

The process has produced satisfactory brightening on the variousstainless steels, silicon steels, ordinary carbon steels, aluminum,nickel, copper, brass, Monel metal, cadmium, and alloys thereof.

Because of its cheapness and convenience I prefer to useortho-phosphoric acid, although in many cases satisfactory results areobtained by the use of metaor pyro-phosphoric acid. Therefore, wheneverI speak of phosphoric acid I intend to include not only the preferredorthophosphoric acid but also the other two forms insofar as they areequivalents.

While I have pointed out certain specific examples of the composition ofthe bath and certain specific examples of current densities,temperatures and polishing times, it is to be understood that these areexemplary only, and that modifications will present themselves to thoseskilled in the art. I therefore do not intend to limit myself except aspointed out in the claims which follow.

Having now fully described my invention, what I claim as new and desireto secure by Letters Patent is:

1. An electrolyte for electrolytically bright polishing stainless steelwhich, as initially produced, consists of from substantially 57.4% tosubstantially 82.9% by weight of the substance phosphoric acid, fromsubstantially 2.5% to substantially 3 .5% by weight of normal butylalcohol, and from substantially 10.1% to substantially 21.4% by weightof water.

2. A process of anodically polishing metals and alloys which under theinfluence of the electric current are not completely insoluble inphosphoric acid, which are low in metalloids, and which do not formbath-insoluble reaction prodnets with phosphoric acid, said processcomprising preparing an electrolyte consisting of from substantially57.4% to substantially 82.9% by weight of, the substance phosphoricacid, from substantially 10.1% to substantially 21.4% of water byweight, and from substantially 2.5% to substantially 32.5% by weight ofan organic substance selected from the group consisting of normal butylalcohol, secondary butyl alcohol, butyl acetate, normal amyl alcohol,secondaryamyl alcohol, and amyl acetate, and electrolyzing therein suchmetals or alloys.

3. The process of claim 2' wherein the current density is fromsubstantially 1 to substantially 5 amperes per treated square inch ofthe said article and wherein the electrolyte is maintained at atemperature of substantially F. to substantially F.

4. In a process of anodically polishing stainless steels, the steps ofpreparing an electrolyte consisting of from substantially 57.4% tosubstantially 82.9% by weight of the substance phosphoric acid, fromsubstantially 10.1% to substantially 21.4% of water by weight, and fromsubstantially 2.5% to substantially 32.5% by weight of an organicsubstance selected from the group consisting of normal butyl alcohol,secondary I butyl alcohol, butyl acetate, normal amyl alcohol, secondaryamyl alcohol, and amyl acetate, and electrolyzing therein a stainlesssteel article.

5. The process of claim 4 wherein the current density is fromsubstantially 1 to substantially 5 amperes per treated square inch ofthe said article and wherein the electrolyte is maintained at atemperature of substantially 120 F. to substantially 180 F.

HAROLD I. WHITE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 571,531 Langhans Nov. 17, 18962,315,695 Faust Apr. 6, 1943 2,294,227 Delaplace et al Aug. 25, 19422,040,618 Mason ct a1 May 12,, 1936 FOREIGN PATENTS Number Country Date707,526 France Apr. 14, 1931 686,131 Germany Jan. 3, 1940 682,248Germany Oct. 20, 1939 404,819 Great Britain Jan. 25, 1934 OTHERREFERENCES "Transactions of the Electrochemical Society, volume 78(1940), pages 265 thru 272; article by Uhlig, entitled "ElectrolyticPolishing of Stainless Steel.

